Thermochromatic inks are inks that change color in reaction to temperatures. They can be formulated such that they change from any color to any color, including turning from opaque or translucent colors, or turning to opaque or translucent colors, in reaction to a wide variety of temperatures. They can be formulated such that they change colors at a variety of temperatures, such that a particular formulation can have several different temperature transition points, each with a particular color change from one particular color to another. They can be mixed with non-thermochromatic inks, such that blending of the constant color of the non-thermochromatic ink with the temperature-dependent color of the thermochromatic ink occurs.
Thermochromatic inks are formulated using chemical compounds that have thermochromatic properties. These thermochromatic compounds generally come in two forms. The first common form is thermochromatic liquid crystals. The color change of these substances is dependent on selective reflection of certain wavelengths by the crystal structure of the material. As the temperature changes, the crystal structure can change between a low-temperature crystal structure, an anisotropic chiral or twisted nematic crystal structure, and a high-temperature isotropic liquid phase. These compounds find application in thermochromatic inks in which color change occurs at a specific temperature, and can sometimes be found in, for example, thermometers for room, refrigerator, aquarium, and medical use, and also in indicators that show the level of propane in pressurized tanks.
The second common form of thermochromatic compounds are leuco dyes. These substances work by having more than one molecular form, which can change via protonation. The different forms have different pi-bond chromophores, thus they exhibit different colors. By making the protonation of the substance dependent on a desired temperature, for example by placing the substance with a weak acid in a solvent that melts at that desired temperature, the color of the substance can be dependent on temperature as well. Leuco dyes change color at a less narrow and less predictable temperature than liquid crystals. Leuco dues have found application in clothing that changes color depending on the heat of the wearer. Both liquid crystals and leuco dyes are usually found in the form of microcapsules both in inks and in other applications.
Some thermochromatic compounds are formulated such that they can reversibly change color, such that they change to a particular color after crossing a particular temperature, and then can change back to the original color when crossing back over that particular temperature. Other thermochromatic compounds are formulated such that they retain a coloration change once the coloration change has occurred.
Some of the applications for thermochromatic compounds include mood rings, measuring the temperature of a liquid in a coffee cup, and indicating when to take a bottle of syrup out of the microwave by the color of a temperature-sensitive label. Other uses include taking someone's temperature with a piece of paper on the forehead, labels for boxes of perishable foods, and battery level monitors.
Toys benefit people in both the educational process and in providing amusement. This is particularly so in the case of children, because they need stimulation to develop their minds and to expend their energy. Toys that have interesting colors or other properties can be fun or educational. For example, devices that allow one to make and show drawn images are useful as toys, as drawing tools for people of any age, and for commercial applications such as showing the day's menu in a restaurant setting. Having such a device that is backlit can enhance the visual appeal of the drawing.
Backlit image display devices are valuable means of displaying images. Such devices can enhance the visual appearance of an image, an effect that can be even more pronounced in dimly lit settings. When shown by such a device, an image with colored or translucent or clear markings or background can be enhanced such that the backlight shows at least partially through and illuminates the markings or background. Simply placing incandescent or florescent light bulbs behind an image is one way to accomplish this effect. Energy- and space-efficient means of illuminating an image from the back are needed.
Coloring and drawing is a creative and satisfying activity for both adults and children. Items for coloring and drawing with pre-printed markings such as coloring books or dot-to-dots are interesting and satisfying ways for people to experience coloring and drawing. After these items are used, they cannot generally be used again. Also, the pre-printed markings in these items that help guide the user can remain visible after the user is finished, which can detract from the visual aesthetics of the resulting colored or drawn design.